1. Field of the Invention
Ths present invention relates to an electron microscope, and more particularly to an electron microscope in which the astigmatism of the magnifying lens system can be readily confirmed.
2. Description of the Prior Art
In an electron microscope which irradiates a specimen with an electron beam and magnifies the electron beam which has passed through the specimen in angular spread to project an enlarged image of the specimen onto a viewing screen, the astigmatism of the imaging lens system, namely, the magnifying lens system exerts a great influence upon the resolving power of the electron microscope. Specifically, the astigmatism of the objective lens which has a large magnification is an important problem. Accordingly, in designing an electron microscope, a great effort is spent on avoiding astigmatism. However, it is extremely difficult to completely eliminate the astigmatism. Therefore, electron microscopes are usually provided with means for correcting astigmatism.
In order to correct astigmatism, the presence of astigmatism has to be confirmed, and the following Fresnel fringe method has been generally employed to confirm the presence of astigmatism. That is, when the image of a hole provided in a collodion film is observed on the viewing screen of the electron microscope, a Fresnel fringe is simultaneously observed which is generated due to electron diffraction in the periphery of the image of the hole. In this case, when the magnifying lens system produces an astigmatism, the width of the Fresnel fringe in the direction of astigmatism differs from that in the direction perpendicular to that of astigmatism. Accordingly, the presence of astigmatism can be confirmed by the difference in width of the Fresnel fringe formed on the viewing screen. Such a method for confirming the astigmatism is generally called the Fresnel fringe method.
In the Fresnel fringe method, however, it is required to compare the widths of the Fresnel fringe in the orthogonal directions, and to judge whether the widths are different from each other or not. Since the difference between the above widths is very small, it is very difficult to confirm the presence of astigmatism in this way. For example, when a hole in a collodion film was irradiated with an electron beam accelerated by a voltage of 50 KV, the Fresnel fringe in the periphery of the image of the hole had a maximum width of 40 A in the direction of astigmatism and a minimum width of 35 A in the direction perpendicular to that of astigmatism. These values were measured by a microphotometer. The result of calculation based upon the above values shows that the astigmatic spacing is equal to 0.2 .mu.m. When a lens system having, for example, a magnification of 10.sup.5 is employed, a difference in width of the Fresnel fringe which is equal to 5 A, is observed as a difference of 0.05 mm on the viewing screen. However, it is practically impossible to confirm by the naked eye the above difference of 0.05 mm in the orthogonal directions.